Physics Special Colloquium, January 11, 2005

From the Fast Lane to Rush Hour: Examining the Forces that Power Cell Motility and Rearrangements

Arpita Upadhyaya

MIT

A fundamental attribute of living cells is their ability to move. One challenge for physicists is to understand how biology exploits physical processes to cause movement. The movement of single cells can be driven by different physical mechanisms such as polymerization or conformational changes of proteins. At a larger length scale and an increasing level of complexity, cells in tissues move in a coordinated manner to create coherent structures. In this talk, I shall describe our studies of three model systems to investigate the physical basis of motion and rearrangement in biology.

Vorticella, one of the most powerful cellular machines, is a single celled organism with a cell body attached to a substrate by a slender stalk which contains a rod-like polymeric structure - the spasmoneme. Vorticella motility is characterized by an extremely rapid contraction which is powered by the entropic collapse of the spasmoneme. We have conducted high-speed imaging experiments to study the dynamics of contraction.

The polymerization of the protein, actin, appears to be the source of the propulsive force for eukaryotic cell motion. While the alphabet soup of proteins that initiate and control actin polymerization is being scrupulously characterized, it is not clear how this generates a force to push. I will describe our attempts in reconstructing motility using phospholipid vesicles as model cell membranes and understanding how actin polymerization exerts a force.

The differential adhesion between cells is believed to be the major driving force behind the formation of tissues. The idea is that an aggregate of cells minimizes the overall adhesive energy between cell surfaces. We demonstrate in a model experimental system of growing yeast colonies, that there exist conditions where a slowly growing tissue does not minimize this adhesive energy. The instability of a spherical shape is caused by the competition between elastic and surface energies.

10:30 a.m., Smith Laboratory, Room 1094

Refreshments served in Smith 1094 at 10:00 a.m.